The present invention relates to an adsorbent for a hydrocarbon. Further, the present invention relates to a catalyst for removing nitrogen oxide in an exhaust gas discharged from an internal combustion engine of e.g. an automobile, and a method for purifying an exhaust gas.
For purification of an exhaust gas containing hydrocarbons discharged from an internal combustion engine of e.g. an automobile, a method has been practically used in which a three way catalyst is contacted with the exhaust gas. However, when the exhaust gas temperature is low at the start up of an engine, the hydrocarbons are likely to be discharged as they are i.e. without being purified by the catalyst.
Therefore, for purification of hydrocarbons from an exhaust gas at a low temperature, JP-A-2-135126 proposes an exhaust gas-purifying apparatus employing an adsorbent for hydrocarbons, having at least one metal supported on a part of a monolithic substrate having Y-type zeolite and mordenite-structured zeolite coated thereon, for the purpose of adsorbing and purifying hydrocarbons. Further, many other adsorbents for hydrocarbons have been proposed wherein zeolites are used as constituting components.
For example, JP-A-5-31359 proposes a zeolite having SiO2/Al2O3 molar ratio of at least 40; JP-A-7-213910 proposes at least one zeolite selected from the group consisting of a mordenite, xcex2-type zeolite and ZSM-5, having a SiO2/Al2O3 molar ratio of from 50 to 2000, and USY having a SiO2/Al2O molar ratio of from 50 to 300; JP-A-8-10566 proposes a zeolite selected from ZSM-5, USY and xcex2-type zeolite, having a SiO2/Al2O3 molar ratio of at least 40; and JP-A-8-224449 proposes a zeolite having a weight ratio of silica to alumina of larger than 300, wherein a preferred zeolite is ZSM-5, zeolite Y, ZSM-11, xcex2-type zeolite or silicalite.
Further, JP-A-8-229386 discloses a xcex2-type zeolite prepared by adding seed crystals of xcex2-type zeolite to a mixed gel, followed by heating at a mild temperature rising rate of from about 28 to 55xc2x0 C./hr, and JP-A-9-38485 proposes, as a constituting component for an adsorbent for hydrocarbons, a xcex2-type zeolite which has been subjected to aluminum-removing treatment so that the residual aluminum is less than 20%, whereby the degree of aluminum removal is set so that the specific surface area is maintained at a level of at least 500 m2/g
Each of such methods for adsorbing and removing hydrocarbons by means of these adsorbents, is one wherein a hydrocarbon contained in an exhaust gas is once adsorbed on an adsorbent at a low temperature during the start up of an engine and kept adsorbed to a temperature at which an exhaust gas-purifying catalyst will operate, and the hydrocarbon desorbed from the adsorbent in a temperature range higher than said temperature, is purified by the exhaust gas-purifying catalyst.
At present, nitrogen oxide, carbon monoxide and hydrocarbons, which are hazardous to human bodies, in an exhaust gas discharged from a gasoline engine, are removed by a three way catalyst having Pt, Pd or Rh included on a support. In recent years, it has been attempted to disseminate a gasoline engine of a lean-burn system or a direct injection type combustion system or a diesel engine in order to reduce the discharge of carbon dioxide in view of a resource problem or an environmental problem due to warming up of the earth. The exhaust gas from such an engine contains excessive oxygen, and it is accordingly difficult to remove nitrogen oxide by a conventional three way catalyst.
As a method for removing nitrogen oxide from an exhaust gas containing excessive oxygen, there has heretofore been known a method for selective-reduction on V2O5/TiO2 wherein ammonia is used as a reducing agent, or a method for absorption into an alkaline solution. However, ammonia is a poisonous substance as designated by law and is highly dangerous, and it is difficult to apply such a method to an automobile which is a mobile source. On the other hand, it is difficult to apply the absorption method to a mobile source such as a common automobile, from the viewpoint of the operation efficiency in e.g. supplementation or treatment of the alkaline solution.
For purification of an exhaust gas in an excessive oxygen state, many exhaust gas-purifying catalysts have been proposed, including an exhaust gas-purifying catalyst consisting of a zeolite ion-exchanged with at least one metal selected from Pt, Pd, Rh, Ir and Ru (JP-A-1-135541). The catalyst proposed in JP-A-1-135541, is one which reduces and removes nitrogen oxide by means of a reducing gas such as hydrogen or hydrocarbons contained in the exhaust gas.
Further, for the purpose of removing nitrogen oxide in an exhaust gas and suppressing discharge of hydrocarbons, it has been proposed that a hydrocarbon in an exhaust gas is adsorbed on an adsorbent at a low temperature, and by utilizing the hydrocarbon which desorbs from the adsorbent as the exhaust gas temperature rises, the performance for the removal of nitrogen oxide is further improved. The following catalysts have heretofore been proposed as exhaust gas-purifying catalysts, wherein an adsorbent for a hydrocarbon and a nitrogen oxide-removing catalyst are used in combination.
JP-A-2-56247 proposes an exhaust gas-purifying catalyst having a first catalyst layer composed mainly of a zeolite formed on a substrate and a second catalyst layer composed mainly of a noble metal catalyst having an redox ability formed thereon, as a catalyst whereby a hydrocarbon is selectively adsorbed on the zeolite in a cold state and in an air/fuel ratio rich state, and the hydrocarbon which is desorbed from the zeolite as the exhaust gas temperature rises, and nitrogen oxide, carbon monoxide and hydrocarbons in the exhaust gas are purified. JP-A-5-293380 proposes an exhaust gas-purifying catalyst comprising a catalyst containing at least Pt on porous material and on aluminosilicate, which has solid acidity and molecular sieve function, containing at least one metal selected from alkali metals and alkaline earth metals.
Further, JP-A-8-24655 proposes an exhaust gas-purifying catalyst prepared by mixing a hydrocarbon adsorbent which adsorbs a hydrocarbon in an exhaust gas and desorbs the adsorbed hydrocarbon at a temperature higher than a certain level, with a NOx catalyst having a metal supported on a crystalline metal-containing silicate, which purifies nitrogen oxide in the exhaust gas in the presence of a hydrocarbon; and JP-A-8-164338 proposes an exhaust gas-purifying catalyst, wherein a hydrocarbon adsorbent made of an inorganic crystalline molecular sieve, is supported on a substrate, a first catalyst layer comprising Pd as a catalyst metal is formed on the surface of the hydrocarbon adsorbent particles, a rare earth oxide layer composed mainly of a rare earth oxide, is formed on the first catalyst layer, and a second catalyst layer comprising at least one of Pt and Rh as a catalyst, is formed on the rare earth oxide layer.
Further, JP-A-9-872 proposes an exhaust gas-purifying system wherein an adsorbent having a hydrocarbon-adsorbing ability and a cold ignition catalytic composition comprising a noble metal and a material having electron donative and/or nitrogen dioxide absorptive and desorptive activities, are disposed in an exhaust pipe of an internal combustion engine.
In recent years, an attention has been drawn to a problem of environmental pollution due to discharge of hydrocarbons, and it is desired to improve the technology for removing such hydrocarbons.
When a zeolite is used as an adsorbent, the adsorption characteristics of a hydrocarbon are influenced substantially by the type of the hydrocarbon and the matrix structure of the zeolite. With respect to the adsorption characteristics of a hydrocarbon having a small carbon number, as the molecular diameter is small, its diffusion and migration into zeolite pores are easy, and adsorption is likewise easy. However, due to the easiness in migration of the hydrocarbon, desorption also tends to be easy, and purification tends to be inadequate, since the hydrocarbon tends to be desorbed at a temperature lower than the temperature at which the catalyst for purifying the hydrocarbon, represented by the three way catalyst, will operate. On the other hand, with respect to the adsorption characteristics of a hydrocarbon having a large carbon number, a hydrocarbon having a molecular diameter larger than the pore diameter of the zeolite, can hardly diffuse or migrate into the pores, whereby the pores of the zeolite can not function sufficiently as adsorption sites. Thus, the amount of the hydrocarbon adsorbed decreases, and the hydrocarbon will be discharged as it is i.e. without being adequately purified.
With respect to the adsorbents which have been proposed heretofore, studies have been made on adsorption characteristics for lower hydrocarbons such as ethylene and propylene or aromatic compounds such as toluene and hydrocarbons discharged from gasoline engines. However, details of adsorption characteristics relating to hydrocarbons having at least 7 carbon atoms, particularly hydrocarbons such as straight chain paraffins and polycyclic aromatic compounds, as constituting components of light oil, have not yet clearly been understood.
Further, the exhaust gas-purifying catalyst proposed as described above, exhibits some improvement in the nitrogen oxide removal activity by an adsorbed hydrocarbon, by the combination with the adsorbent for a hydrocarbon, but such an effect has been inadequate, and further improvement in the performance has been desired.
Further, the temperature of an exhaust gas from an internal combustion engine is possible to reach a level of at least 600xc2x0 C. Accordingly, the adsorbent for a hydrocarbon and the exhaust gas-purifying catalyst are required to have high thermal resistance, so that the adsorption performance and the purifying activity will not decrease even when the adsorbent and the exhaust gas-purifying catalyst are exposed to such a high temperature.
It is an object of the present invention to solve the above mentioned problems of the prior art and to provide an adsorbent which has a high ability of adsorbing a hydrocarbon and which has sufficient thermal resistance, and a method for adsorbing and removing a hydrocarbon contained in a gas, by means of such an adsorbent.
Another object of the present invention is to provide an exhaust gas-purifying catalyst employing the adsorbent of the present invention and a nitrogen oxide-removing catalyst, and a method for purifying an exhaust gas.
The present inventors have conducted an extensive study on the above problems and as a result, have found that an adsorbent for a hydrocarbon, comprising a xcex2-type zeolite having high crystallinity, has high thermal resistance, and an exhaust gas-purifying catalyst comprising a combination of such an adsorbent and a catalyst which is excellent in durability and a nitrogen oxide-removing performance, has a high nitrogen oxide-removing activity and is excellent in durability with little decrease in the activity even after being exposed to a high temperature. The present invention has been accomplished on the basis of this discovery.
Namely, the present invention provides an adsorbent for a hydrocarbon, comprising a calcined xcex2-type zeolite showing a powder X-ray diffraction such that the sum of X-ray diffraction intensities at lattice spacings d=1.15xc2x10.03 nm and d=0.397xc2x10.01 nm, is at least 90% of the diffraction intensity at d=0.346xc2x10.01 nm of Catalysis Society reference catalyst JRC-Z-HM-20(3).
The present invention also provides a method for adsorbing and removing a hydrocarbon in a gas phase, which comprises contacting such an adsorbent for a hydrocarbon, with the gas phase.
Further, the present invention provides an exhaust gas-purifying catalyst comprising such an adsorbent for a hydrocarbon and a nitrogen oxide-removing catalyst having an active metal contained in a porous material support.
Still further, the present invention provides a method for purifying an exhaust gas, which comprises contacting such an exhaust gas-purifying catalyst with the exhaust gas.
Now, the present invention will be described in detail with reference to the preferred embodiments.
It is essential that the adsorbent for a hydrocarbon of the present invention comprises a xcex2-type zeolite. The xcex2-type zeolite has a composition represented by the formula:
xMn/2O.Al2O3.ySiO2.zH2O
wherein n is an atomic valence of cation M, x is a number within a range of from 0 to 2.5, y is a number of at least 1, and z is a number of at least 0. The structure is disclosed in ZEOLITES, 12(5) 58-59 (1992) and JP-A-5-201722, and it is known as a zeolite having 12-membered ring pores and having a crystal structure represented by the X-ray diffraction as shown in Table 1.
The xcex2-type zeolite constituting the adsorbent of the present invention is required to have a crystallinity such that the sum of X-ray diffraction intensities at lattice spacings d=1.15xc2x10.03 nm and d=0.397xc2x10.01 nm in the powder X-ray diffraction, is at least 90% of the diffraction intensity at d=0.346xc2x10.01 nm of Catalysis Society reference catalyst JRC-Z-HM-20(3), in a state after calcination to remove an organic compound incorporated during the preparation of the zeolite. The crystallinity of a zeolite can usually be evaluated by X-ray diffraction. In general, the crystallinity by X-ray diffraction can be evaluated by the peak intensity (height) of a detected diffraction peak and the peak area. When the crystallinity of zeolite is high, the detection intensities of diffraction peaks attributable to the zeolite are high, and when the crystallinity is low in a case where impurities such as amorphous components are co-existent, the diffraction peaks tend to be small. Namely, the xcex2-type zeolite of the present invention contains substantially no impurities such as amorphous components and has high crystallinity, whereby the adsorption capacity increases, and the thermal resistance is improved.
The SiO2/Al2O3 molar ratio of the xcex2-type zeolite is not particularly limited, but it is preferably from 10 to 1000, more preferably from 20 to 500, in order to increase the thermal resistance and the adsorption characteristics for a hydrocarbon.
Further, the crystal particle size of the xcex2-type zeolite to be used in the present invention, is not particularly limited. However, in order to increase the thermal resistance, it is preferred that the average particle size is from 3 to 10 xcexcm, and the zeolite has a particle size distribution such that particles of at least 1 xcexcm constitutes at least 80%. The particle size of the zeolite can be measured by an analytical instrument such as an electron microscope or a Caulter counter, and it can be evaluated by a gauss-type distribution represented by the occupying proportion of particles with particle sizes within a described range, and the average particle size calculated from such a particle size distribution.
Now, a process for producing the xcex2-type zeolite will be described. The xcex2-type zeolite of the present invention may be one having the above described crystallinity, and the process for its production is not particularly limited. For example, it can be produced by a hydrothermal synthesis wherein an aqueous reaction slurry employing e.g. tetraethylammonium as a template agent as disclosed in JP-A-5-201722, is used as a starting material, and the starting material is heated for crystallization. It is further preferred to employ a process as disclosed in JP-A-9-175818 in which a powdery starting material composition dried at a temperature of at least 50xc2x0 C., is contacted with steam spontaneously generated at a temperature of from 80 to 200xc2x0 C., without directly contacting it with water of a liquid phase, whereby a xcex2-type zeolite having higher crystallinity can be obtained.
In the preparation of a xcex2-type zeolite as disclosed in JP-A-9-175818, the starting material composition comprises an alumina source, a silica source, an alkali source and tetraethylammonium ions. As the alumina source, aluminum sulfate, sodium aluminate, aluminum hydroxide or aluminosilicate gel may, for example, be used, and as the silica source, colloidal silica, amorphous silica, sodium silicate or aluminosilicate gel may, for example, be used. Preferred is one in the form whereby it can adequately uniformly be mixed with other components.
As the alkali source, an alkali component in sodium hydroxide, sodium aluminate or sodium silicate, or an alkali component in an aluminosilicate gel, may, for example, be preferably employed. Further, potassium hydroxide or potassium aluminate may, for example, be used. For the tetraethylammonium ions, it is usual to employ tetraethylammonium hydroxide.
These materials are sufficiently mixed in the presence of water to obtain a uniform slurry. The amount of water used here is not particularly limited. Then, this uniform slurry is dried at a temperature of at least 50xc2x0 C. to obtain a powdery starting material composition. The upper limit for the drying temperature is not particularly limited, but it is preferably within a temperature range in which water does not boil. The drying is uniformly carried out until the equilibrium water content is reached at that temperature. If the drying temperature is lower than 50xc2x0 C., the water content of the powdery starting material composition tends to be high, whereby the crystallinity tends to be low. The method for drying the uniform slurry is not particularly limited, but it is preferred to dry the aqueous slurry of the starting material mixture with stirring.
The chemical composition of the powdery starting material composition thereby obtained, is represented by the following molar ratios of oxides:
SiO2/Al2O3=10 to 1000
M2O/SiO2=0 to 0.4
TEA2O/SiO2=0.1 to 1.0
wherein M is an alkali metal, and TEA is tetraethylammonium, preferably:
SiO2/Al2O3=20 to 600
M2O/SiO2=0 to 0.1
TEA2O/SiO2=0.15 to 0.25
This powdery starting material composition is put into a sealed container and crystallized by contacting it with steam spontaneously formed at a predetermined temperature, without directly contacting it with water of a liquid phase.
The crystallizing temperature is preferably within a range of from 80 to 200xc2x0 C. If it is lower than 80xc2x0 C., the crystallization speed is very slow, and it takes a long time for crystallization, such being economically disadvantageous. On the other hand, if the temperature is higher than 200xc2x0 C., decomposition of tetraethylammonium ions tends to be vigorous, whereby it tends to be difficult to obtain a xcex2-type zeolite having high crystallinity.
The xcex2-type zeolite produced by the above described process is used as a calcined product. Calcination of the synthesized zeolite is carried out to remove tetraethylammonium ions contained in the zeolite during the synthesis, and it is carried out under a condition hereby tetraethylammonium ions are sufficiently removed. n general, the calcination is carried out at a temperature of from 550 to 1000xc2x0 C. for a calcination time of from 0.5 to 10 hours. The calcination atmosphere may be air, nitrogen, or air or nitrogen containing an oxidative gas. Ions such as Na in the xcex2-type zeolite may be treated with an ammonium salt or a mineral acid, so that the zeolite can be used in the H-type or the NH4-type.
The adsorbent of the present invention may be one having a transition metal contained in the above P-type zeolite, whereby the performance for adsorbing a hydrocarbon can be further improved. When a transition metal is to be incorporated, the transition metal is not particularly limited and may, for example, be an element of Group IIIa, IVa, VIa, VIIa, VIII, Ib or IIb of the Periodic Table. At least one of these transition metals may be incorporated. As the transition metal, Cu and/or Ag is preferred. The method for incorporating the transition metal is not particularly limited, and any conventional method may suitably be employed. For example, it is possible to employ an ion exchange method, an impregnation-supporting method, an evaporation-to-dryness method, a dipping method or a solid phase exchange method. The salt to be used for incorporating the transition metal is not particularly limited, and a salt such as a nitrate, a sulfate, an acetate, an oxalate or an ammine complex salt, may, for example, be used. If more than one transition metal is to be incorporated, such transition metals may sequentially be incorporated, or may simultaneously be incorporated.
The content of the transition metal contained in the adsorbent prepared by the above process, is preferably within a range of from 0.1 to 20 wt %, more preferably from 0.2 to 10 wt %, based on the total amount of the transition metal and the xcex2-type zeolite, in order to obtain an adequate performance for adsorbing a hydrocarbon.
As described in the foregoing, the adsorbent for a hydrocarbon of the present invention can be produced.
The adsorbent of the present invention can be used as mixed and molded with a binder such as silica, alumina or a clay mineral. The clay mineral to be used at the time of molding, may, for example, be a clay mineral such as kaolin, attapulgite, montmorillonite, bentonite, allophane or sepiolite. Further, the adsorbent for a hydrocarbon of the present invention may be used as wash-coated on a honeycomb substrate made of cordierite or metal.
Adsorption and removal of a hydrocarbon in a gas phase, can be carried out by contacting the adsorbent of the present invention with the gas phase. This gas phase is not particularly limited, and the present invention can be applied, for example, to a gas phase containing a hydrocarbon, such as the atmosphere or an exhaust gas. Further, the present invention is effective also in a case where the gas phase contains carbon monoxide, carbon dioxide, hydrogen, oxygen, nitrogen, nitrogen oxide, sulfur oxide or water in addition to the hydrocarbon.
The types of hydrocarbons to be adsorbed by the adsorbent include, for example, unsaturated hydrocarbons (olefins) such as ethylene, propylene, butene and 1-hexadecene, saturated hydrocarbons (paraffins) such as methane, ethane, propane, n-decane and n-hexadecane, aromatic hydrocarbons such as benzene, toluene, naphthalene and anthracene, and branched hydrocarbons such as 2-methylpropane and isopropylbenzene. Further, as derivatives of the above hydrocarbons, oxygen-containing organic compounds such as ketones and aldehydes, and nitrogen-containing organic compounds such as amines, may also be adsorbed. Namely, the gas phase to be treated by the adsorbent of the present invention is one containing at least one type of these hydrocarbons. The hydrocarbon to be adsorbed, is preferably a straight chain paraffin having at least 7 carbon atoms, a straight chain olefin having at least 7 carbon atoms and/or a polycyclic aromatic compound, more preferably a straight chain paraffin having from 10 to 18 carbon atoms, or a straight chain olefin having from 10 to 18 carbons atoms.
The concentration of the hydrocarbon in the gas phase is not particularly limited, but it is preferably from 0.001 to 5 vol %, more preferably from 0.005 to 3 vol %, as calculated as methane. The concentrations of various components other than the hydrocarbon are also not particularly limited, and, for example, CO=0 to 1 vol %, CO2=0 to 10 vol %, O2=0 to 20 vol %, nitrogen oxide=0 to 10 vol %, sulfur oxide=0 to 0.05 vol %, and H2O=0 to 15 vol %.
The space velocity and the temperature at the time of adsorbing and removing the hydrocarbon, are not particularly limited. However, the space velocity is preferably from 100 to 500,000 hrxe2x88x921, and the temperature is preferably from xe2x88x9230 to 250xc2x0 C.
The exhaust gas-purifying catalyst of the present invention comprises the adsorbent component for a hydrocarbon of the present invention and a catalyst component for removing nitrogen oxide.
Now, the catalyst for removing nitrogen oxide, constituting the exhaust gas-purifying catalyst of the present invention, will be described.
The nitrogen oxide-removing catalyst of the present invention is a catalyst having an activity to remove nitrogen oxide and having an active metal contained in a porous material, preferably one showing the activity also under an oxygen excessive exhaust gas. A catalyst is preferred which has at least one noble metal selected from Pt, Pd, Ir and Rh incorporated as the active metal on a porous material, and the average particle size of the noble metal is preferably at least 10 nm. If the average particle size of the noble metal is at least 10 nm, the hydrocarbon in the exhaust gas and the hydrocarbon supplied from the adsorbent, can effectively be utilized, and the nitrogen oxide removal activity can be improved. The reason for such improvement is not clearly understood. However, it is believed that the activity for reducing and removing nitrogen oxide by the hydrocarbon can be increased by a proper balance of the reaction for reducing and removing nitrogen oxide and a reaction for complete combustion of the hydrocarbon, or by formation of special active sites. Further, it is considered that when the size of noble metal particles is at least 10 nm, the noble metal particles are stabilized, whereby the thermal stability will be improved. As the noble metal, Pt is preferably employed from the viewpoint of the activity and the thermal stability.
The porous material is not particularly limited. For example, an oxide such as alumina, silica, zirconia or titania, a composite oxide such as silica-alumina, silica-zirconia or silica-titania, or zeolite, may be used. Among them, alumina or zeolite is preferred from the viewpoint of the activity and durability.
The method for incorporating the transition metal to the porous material is not particularly limited, and it is possible to employ a conventional method such as an impregnation-supporting method, an evaporation-to-dryness method, a physical mixing method or an ion exchange method. The compound to be used for incorporating the active metal is also not particularly limited, and a nitrate, a sulfate, a chloride or an ammine complex salt, may, for example, be used. The amount of the active metal incorporated to the porous material, is not particularly limited, but it is preferably at least 0.5 wt %, relative to the porous material, in order to obtain an adequate nitrogen oxide removal activity. Further, the noble metal is preferably at most 10 wt %, relative to the porous material from the viewpoint of the production cost, so long as the effect corresponding to the incorporated amount is obtainable.
The porous material having the active metal incorporated, is preferably subjected to thermal treatment to stabilize the active metal. To stabilize the active metal with a particle size of 10 nm, it is preferably treated at a temperature of from 400 to 900xc2x0 C. The atmosphere for the thermal treatment is not particularly limited, and treatment can be carried out in air, nitrogen, hydrogen or a gas containing hydrogen.
As described in the foregoing, the nitrogen oxide-removing catalyst of the present invention can be produced.
The hydrocarbon adsorbent and the nitrogen oxide-removing catalyst constituting the exhaust gas-purifying catalyst of the present invention, can be used as mixed and molded with a binder such as silica, alumina or a clay mineral. The clay mineral to be used at the time of the molding, may, for example, be a clay mineral such as kaolin, attapulgite, montmorillonite, bentonite, allophane or sepiolite. Further, the hydrocarbon adsorbent of the present invention and the nitrogen oxide-removing catalyst may be used as wash-coated on a honeycomb substrate made of cordierite or metal.
The manner of combining the two components to prepare the exhaust gas-purifying catalyst is not particularly limited. However, in order to have the hydrocarbon adsorbed on the xcex2-type zeolite and to utilize the hydrocarbon desorbing from the adsorbent effectively for the nitrogen oxide removal reaction, it is preferred that the hydrocarbon desorbing from the xcex2-type zeolite is supplied efficiently to the nitrogen oxide-removing catalyst, and the temperature for desorption of the hydrocarbon agrees to the operation temperature of the nitrogen oxide-removing catalyst. To obtain such a state, the nitrogen oxide-removing catalyst may be selected to meet with the temperature for desorption of the hydrocarbon from the xcex2-type zeolite. However, such an effect can also be obtained by properly adjusting the positions. For example, when the catalyst of the present invention is used as wash-coated on a molded product or on a honeycomb substrate, it is possible to employ a method wherein the xcex2-type zeolite is disposed at the upstream side of the exhaust gas, and the nitrogen oxide-removing catalyst is disposed at the downstream side thereof, a method wherein the xcex2-type zeolite layer and the nitrogen-oxide-removing catalyst layer may be used as laminated to each other, or a method wherein the xcex2-type zeolite and the nitrogen oxide-removing catalyst are employed in admixture in the form of powders or molded products.
As described in the foregoing, the exhaust gas-purifying catalyst of the present invention can be produced by combining the hydrocarbon adsorbent and the nitrogen oxide-removing catalyst.
Removal of the nitrogen oxide from an exhaust gas can be carried out by contacting the above exhaust gas-purifying catalyst with the exhaust gas, and this is effective also for an oxygen excessive exhaust gas. The exhaust gas contains nitrogen oxide and hydrocarbons. Here, the oxygen excessive exhaust gas is meant for an exhaust gas which contains oxygen in excess of the amount of oxygen required to completely oxidize the hydrocarbons, hydrogen and carbon monoxide contained in the exhaust gas. Such an exhaust gas may, for example, be an exhaust gas discharged from an internal combustion engine such as a diesel engine, particularly an exhaust gas from combustion in a large air/fuel ratio. Further, the above exhaust gas may contain hydrocarbons, carbon monoxide, carbon dioxide, hydrogen, sulfur-oxide and water.
The types of hydrocarbons contained in the exhaust gas to be treated by the present invention, are not particularly limited and include, for example, paraffins, olefins, aromatic compounds and their mixtures. Specifically, as the paraffins and olefins, hydrocarbons having from 10 to 20 carbon atoms, may be used, and as the aromatic compounds, benzene, naphthalene and their derivatives may be used. Further, two or more hydrocarbons selected from the above olefins, paraffins and aromatic compounds, may be used as mixed, and light oil, kerosene or gasoline may also be used. The concentrations of the respective components in the exhaust gas are not particularly limited, but they are usually preferably such that nitrogen oxide is from 50 to 2,000 ppm, hydrocarbons are from 10 to 10,000 ppmC (based on carbon), and oxygen is from 0.1 to 20%. Further, the above mentioned suitable hydrocarbon may be incorporated to the exhaust gas, in order to further increase the nitrogen oxide-removing activity.
The space velocity and the temperature of the exhaust gas to be treated, are not particularly limited. However, preferably, the space velocity (based on the volume) is from 500 to 500,000 hrxe2x88x921, and the temperature is from 100 to 800xc2x0 C. More preferably, the space velocity is from 2000 to 300,000 hrxe2x88x921, and the temperature is from 100 to 600xc2x0 C.